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Northwest Iowa Crop Update Newsletter
by Todd Vagts
ISU Extension Crops Specialist
Counties Served:  Carroll, Calhoun, Crawford, Ida, Monona, Pocahontas and Sac.

[Home][Special Topics][Weather Data][Subsoil H20][PDF Info] [ISU Extension][IA State University]

Volume 5, Number 17
Northwest IA Crop Update, July 18, 2005
Print or view this newsletter in  PDF or Microsoft Word format.

In this issue

bullet Rain brings temporary relief
bullet Degree-day accumulation may top 200
bullet Soybeans setting pods
bullet WBC moth flight near peak
bullet Soybean aphids arrive
bullet Predicting soybean aphid population growth

Lack of precipitation intensified the problem of hot temperatures last week across much of west-central Iowa.  I noticed stressed corn plants by late last week.  This moisture stress could be detrimental to potential corn yields as the corn crop is in the very critical pollination stage.   The soybean crop is relatively safe at this point, but the forecasted continued hot and dry conditions could start to limit soybean yield potential as well as the soybean crop nears its critical period of pod fill.  The rain over the weekend brought temporary relief, but more precipitation will be needed over the next two weeks to keep this crop at a “high” yield potential status.  Western bean cutworm moth catch increased dramatically over the weekend, expect peak emergence to occur this week.  Soybean aphids are now showing up in many west-central IA soybean fields, fortunately numbers are presently very low in most fields.  Be sure to be vigilant with this pest over the next three weeks. 

Weather information
Growing Degree Day  Continued above normal temperatures will further boost degree-day accumulations to well above normal levels in west central IA.  Last week’s accumulation of 188 DD was well above the normal of 158.  The forecast of 200 degree-days for the week ahead could be the warmest week the area has seen since late July and early August of 2001 when weekly degree day accumulations were above 200.  Degree-day accumulation continues to move ahead of normal for all planting dates, by next week N.W. Iowa should be nearly 108% of normal compared to the current 105% of normal.  Accumulation and predicted plant phenology stages are shown in Table 1 and Figure 1 and is forecasted through July 24.  More detailed degree-day accumulations by planting date can be obtained at this URL:

Table 1.  Degree-Day Weekly Accumulation
  2005 2004 2003 14-Yr Ave
July 11-17 188 158 152 158
Forcasted July 18-24 200 153 131 162


  Table 2.  2005 Degree-Day % of Average (by planting date)
  10-Apr 15-Apr 20-Apr 25-Apr 30-Apr 05-May 10-May Ave
last week 107% 106% 103% 104% 105% 107% 105% 105%
this week 109% 108% 105% 106% 108% 109% 108% 108%

Crop Management
Corn is pollinating across most of NW Iowa by this time, fortunately much of the area received some moisture Sunday evening which will help alleviate moisture stress, at least temporarily.  Some of the more advanced fields may be finishing pollination and will be in the R2 (blister) stage by mid to late this week.  R2 Stage normally occurs 10-14 days after silking. The forecasted warm temperatures by mid-week will most likely cause stress on the corn crop once again.  Corn is using between 0.25 and 0.33 inches of water per day, the recent rainfall will be used up in a matter of 2 to 3 days depending on the amount of rainfall received. 


Soybean Development has reached R2 to R3 in fields across the region.  R3 is beginning pod growth (3/16 inch pod in one of the upper 4 nodes). 

Pest Management
Western Bean Cutworm  Moth catch increased dramatically in most pheromone traps across N.W. Iowa.  According to degree-day counts, most of N.W. Iowa should be near 50% emerged moths between July 19 and 21.  Seventy-five percent emergence should be reached by July 23 – 25.  View graphed trap catch data and degree-day counts for N.W Iowa at my WBC web page:  For trap catch data around the state and in Missouri and Illinois, go to this web URL:  Read a current article on WBC in this week’s ISU IPM newsletter:

Soybean Aphid  I found my first soybean aphid last Friday in a field just north of Carroll.  I’ve heard reports of aphids now being found throughout most of western IA.  The dry conditions and moisture stressed soybean plant will promote aphid population growth, yet the high temperatures will help to keep population growth in check.  Soybean aphid presence in MN is also increasing, although it doesn’t appear to be as bad as in 2003.  Compare this week’s MN pest report ( with the same week two years ago (  Fields infested and percent of plants infested is less this year compared to 2003.

Estimating aphid population growth.  Use MN’s Sage model to predict aphid population growth over the next seven days (  The University of MN released an aphid population growth calculator called the SAGE model which can be used to help predict the growth potential of an existing aphid population based on current aphid numbers and the 7-day temperature forecast.  The SAGE model can be accessed at this URL:  Click on "Soybean aphid growth estimator v1-2.xls" following the second paragraph.  This is a Microsoft Excel program.

The developers of the SAGE model state that the model will predict an average rate of population build up (expressed as the time required for populations to double). The model will also predict when the population is expected to reach a threshold of 250 aphids per plant. The model is likely to over-estimate aphid populations in the field. Use this model as a guide to determine when to resample a field (preferably at least 2-3 days before threshold is reached).  Seven day temperature forecasts can be obtained from (input your location into the local forecast box).

I ran the SAGE model assuming between 5 and 80 aphids per plant (the field I looked at last week averaged less than one aphid per plant) and the results can be found on my soybean aphid web page:  The model using the 7-day forecast predicts aphid populations to double every 3.4 days at Carroll and every 2.6 days at Pocahontas (this shows the effect of warmer temps at Carroll on aphid population growth).

Aphid Scouting:  Tired of counting all those soybean aphids?  Try using a speed scouting method developed by the University of MN.  For more information and a scouting sheet, go to this URL:  Remember, we are using an economic threshold of 250 aphids per plant with the aphid population increasing and few predators present.   Economic losses typically do not occur until aphid numbers reach 1000 per plant.  Read more about the soybean aphid in this week’s ISU IPM newsletter: 



Print or view this newsletter in PDF or Microsoft Word format.

Todd Vagts
Iowa State University Extension
Field Crops Specialist
1240 D. Heires Avenue 
Carroll, IA 51401 
Office: 712-792-2364; Cell: 712-249-6025;  Fax: 712-792-2366

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This page last updated on 07/18/05

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